The present invention relates to dryer fabrics for use in conventional, single fabric, and dual fabric configurations such as those illustrated in FIGS. 1A, 1B, and 1C, respectively.
The dryer section of a papermaking machine contains a plurality of dryer cylinders which emit heat for drying the wet paper web that has been formed. During the normal operation of the dryer section, such as shown in FIG. 1A, the wet paper web 3 is pressed by dryer fabrics 2a and 2b against the dryer cylinders 1a through 1g. As the dryer fabrics transverse the dryer cylinders and the paper web, the fabrics remove heat from the cylinders and paper web. This heat is lost by radiation from the surface of the bottom fabric as the fabric passes under the cylinders so as to return to the starting point of the dryer section. The heat is further lost by radiation from the surface of the top fabric as it passes over the cylinders and returns to the starting point. The heat emitted by the dryer fabric during its return, therefore, is lost in the process. Rollers 1h and 1i are further support rollers in the embodiment shown in FIG. 1A as well as the embodiments shown in FIGS. 1B and 1C, which are discussed below.
The single fabric configuration, such as shown in FIG. 1B, has been developed in recent years for the dryer section of a papermaking machine; this type of configuration is disclosed in U.S. Pat. No. 3,503,139 to Mahoney. In the configuration of FIG. 1B, a wet paper web 3 is completely supported on the surface of a dryer fabric 2 which traverses dryer cylinders 1a through 1g. Such a configuration helps to eliminate any fluttering of the wet paper web, especially when the machine is driven at high speed. The benefits of employing a single fabric configuration, however, are somewhat offset by the reduction in heat transfer between one row of dryer cylinders and the paper web where the fabric lies between the web and the cylinders. This loss in heat transfer reduces the overall drying rate of the section and can result in production losses.
Alternatively, the reduction in heat transfer is compensated for by increasing the temperature of the drying cylinders. This, however, results in unusually high energy requirements for the papermaking machine, which is obviously undesirable. In addition, in some machines there is significant restraint on the temperature level at which the cylinders can be used.
In a dual, or sandwich, fabric configuration, as shown in FIG. 1C, the paper web 3 travels along a path between dryer fabrics 2a and 2b across dryer cylinders 1a through 1g. The primary advantage to utilizing such a configuration is that the paper web is completely transported thereby significantly decreasing the possibility of web breakage. In such systems, however, the dryer fabrics insulate the paper web from both the top and bottom drying cylinders.
Throughout this application the terms thermal conductivity and emissivity are used. These terms are defined as follows. The term thermal conductivity refers to heat flow per unit of cross sectional area (BTU/HR-FT.sup.2) through the thickness of the fabric subjected to a temperature differential of one degree Fahrenheit from face to back multiplied by the fabric thickness (BTU-IN/HR-FT-.sup.2 -.degree.F.). The term emissivity refers to the ratio of the radiant energy emitted by the surface of the fabric at a given temperature to that emitted by the ideal radiator (i.e., a "black body") at the same temperature.
Dryer fabrics are typically chemically treated with either an acrylic or resorcinol formaldehyde resin. The thermal conductivity properties of such resins are very similar to the monofilament and multifilament fibers and other yarns that are used for weaving the dryer fabrics. The poor thermal conductivity properties of the fibers and yarns coupled together with the openness of the woven structure create fairly poor heat conducting characteristics for the dryer fabrics.
In addition to the insulating properties of the resin treatments and the fibers and yarns themselves, other chemicals that are typically added to the resin treatment also have fairly good insulating properties. Often, various fillers and extenders are added to the resin used for treating the fabric in order to improve properties such as the coefficient of friction, abrasiveness and color/opacity. Examples of such chemicals are titanium dioxide, calcium carbonate, diatomaceous earth, Georgia clay, colored pigments, graphite, carbon black, silica and various ceramics.
Various techniques for coating fabrics with resins have been extensively developed in the prior art. Examples of such coating processes are disclosed in the following U.S. patents: 3,250,662 to N. R. Seaman; 3,519,475 to C. Hoyle et al; and 3,653,961 to L. R. Lefkowitz.
Another patent that may be of interest is U.S. Pat. No. 3,067,779 to J. H. Draper, Jr. This patent discloses utilizing metal strands that are woven into the fabric. Such metal strands serve as electrical conductors through which electricity can pass. Such conductors serve as heating elements for drying the paper being produced.